1. Field of the Invention
The invention is directed to synthetic lubricating oils comprising mixed polymers of ethylene, propylene and a third olefinic monomer such as 1-butene or 1-hexene. The invention is more particularly directed to a novel process of producing high quality synthetic oils by the copolymerization of ethylene, propylene and a third alpha or 1-olefin using a Zeigler-Natta catalyst system followed by a simplified dewaxing technique, urea adduction.
2. Summary of the Prior Art
The preparation of synthetic oils from ethylene and propylene using Ziegler-Natta catalysis is not new. The existing art refers to low viscosity oils which are obtained by thermal cracking and, very likely, deep dewaxing. The art also describes high molecular weight, solid copolymers used as viscosity index improving additives. Thus in general the polymerization of olefinic mixtures using a Ziegler-type catalyst is known.
U.S. Pat. No. 3,923,919 discloses that in producing synthetic hydrocarbon lubricating oils, the attainment of high viscosity index is generally due to the presence of at least 29 mol. % of ethylene to provide an oil with low pour point and high viscosity index.
Other U.S. Pat. Nos. with similar or related disclosures are 3,676,521, 3,737,477 and 3,851,011.
However, none of these patents or the prior art in general discloses or suggests a process wherein urea adduction is incorporated into the polymerization reaction product work-up procedure as a means of producing high quality synthetic oils. The prior art seems to indicate that thermal cracking and deep dewaxing are necessary to produce high quality oils having low viscosity and low pour point.
We have found that copolymers of ethylene and propylene alone produce waxy oils having high VI, high viscosity and high pour point (Table 1) and that conventional, low temperature dewaxing (MEK-toluene) does not adequately reduce viscosity or pour point (semi-solid @ R.T. to --&gt;-10.degree. F.) and seriously reduces finished oil yield (approx. 40% loss versus a 2-4% loss using the system or process described herein). This large 40% loss explains the prior art need for thermal cracking.
Urea adduction is well known and has been used in the past to treat aviation fuels and waxy lubes to preferentially remove wax-like components and thereby improve low temperature flow properties and pour point. The urea dewaxing method was found inappropriate to dewax typical ethylene-propylene copolymer oils (Table 1, ex. 2) due to their high average molecular weight and high wax content. However, the method was found very effective with an ethylene-propylene terpolymer oil in accordance with this invention containing for example either 1-butene or 1-hexene (Table 2, ex. 1, 2 and 4 vs. 3). The use of urea adduction as opposed to prior art dewaxing techniques results in a dramatic and unexpected change in pour point (see Table 2).
Fuel economy and fuel efficient automotive oils are of great interest and importance today. Lubricant viscosity at engine operating conditions is a major contributing factor to this efficiency. Many efforts to formulate fuel-efficient synthetic and semi-synthetic lubricants are currently underway. This invention makes possible the development of semi-synthetic and fully synthetic formulated oils with appreciably reduced viscosity at low pour point. Friction modification through reduced viscosity is, of course, one possible way to achieve improved fuel economy benefits.